Hole-doped cuprate high temperature superconductors have ushered in the modern era of high temperature superconductivity (HTS) and have continued to be at center stage in the field. Extensive studies have been made, many compounds discovered, volumin
ous data compiled, numerous models proposed, many review articles written, and various prototype devices made and tested with better performance than their nonsuperconducting counterparts. The field is indeed vast. We have therefore decided to focus on the major cuprate materials systems that have laid the foundation of HTS science and technology and present several simple scaling laws that show the systematic and universal simplicity amid the complexity of these material systems, while referring readers interested in the HTS physics and devices to the review articles. Developments in the field are mostly presented in chronological order, sometimes with anecdotes, in an attempt to share some of the moments of excitement and despair in the history of HTS with readers, especially the younger ones.
Recent reports of interface-induced superconductivity in the unit-cell films of FeSe on SrTiO3 with a Tc up to ~ 65+-5 K, highest among the Fe-based superconductors and second only to that of the cuprates, have generated great excitement. Here we sho
w results of the first magnetic and resistive investigation on the 1-4 unit-cell FeSe-films on SrTiO3. The samples display the Meissner state below ~ 20 K with a penetration field as low as 0.1 Oe due to large edge effect at 2 K, a mesoscopic superconducting state comprising patches of dimension < 1 micrometer up to 45~55 K and an unusual relaxation of the diamagnetic signal up to 80 K, suggesting possible superconductivity at this high temperature. The observations demonstrate the heterogeneous nature of the superconducting state in these films and offer new insights into the role of interfaces in high temperature superconductivity proposed and future superconducting electronics fabricated.
Due to the similarity to BaFe2As2 and SrFe2As2 the RFe2Si2 (R=La, Y and Lu) system has been proposed as a potential candidate for a new superconducting family containing Fe-Si layers as a structural unit. Various R(Fe1-xMx)2Si2 M=Ni, Mn and Cu) mater
ials were synthesized and measured for their magnetic properties. None of these materials is superconducting down to 5 K. Fe in RFe2Si2 is paramagnetic. A pronounced peak at 232 K was observed in the magnetization curve of YFe2Si2. 57Fe Mossbauer studies confirm the absence of any magnetic ordering at low temperatures. Similar peaks at various temperatures also appear in R(Fe1-xMx)2Si2 samples. Four independent factors affect the peak position and shift it to lower temperatures: (i) the lattice parameters, (ii) the concentration of x, (iii) the applied magnetic field, and (iv) the magnetic nature of M. The peak position is dramatically affected by the magnetic Mn dopants. It is propose that the magnetic peaks observed in RFe2Si2 and in R(Fe1-xMx)2Si2 represent a new nearly ferromagnetic Fermi liquid (NFFL) system and their nature is yet to be determined.
In rare-earth doped single crystalline CaFe2As2, the mysterious small volume fraction which superconducts up to 49 K, much higher than the bulk Tc ~ 30s K, has prompted a long search for a hidden variable that could enhance the Tc by more than 30% in
iron-based superconductors of the same structure. Here we report a chemical, structural, and magnetic study of CaFe2As2 systematically doped with La, Ce, Pr, and Nd. Coincident with the high Tc phase, we find extreme magnetic anisotropy, accompanied by an unexpected doping-independent Tc and equally unexpected superparamagnetic clusters associated with As vacancies. These observations lead us to conjecture that the tantalizing Tc enhancement may be associated with naturally occurring chemical interfaces and may thus provide a new paradigm in the search for superconductors with higher Tc.
We report the discovery of superconductivity at 2.3 K in Zr5Sb3, the first superconducting member in the large compound family of the Mn5Si3-structure type. Transport, magnetic, and calorimetric measurements and band structure calculations show it to
be a phonon-mediated BCS superconductor, with a relatively large density of states at the Fermi level associated with the d-electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr5Sn3. More superconductors with even higher transition temperatures are expected to be found in this family of compounds.
The observation of non-bulk superconductivity with an unexpectedly high onset transition temperature Tc up to ~ 49 K in non-superconducting single crystalline CaFe2As2 upon rare-earth doping has raised interesting questions concerning its origin. Sev
eral possibilities including interfacial mechanism have been proposed to account for the unusual observation. In an attempt to differentiate such propositions, we have carried out a systematic compositional and magnetic study on single crystals of nominal (Ca1-xPrx)Fe2As2 with 0 <= x <= 0.13 throughout the solubility range of Pr, which covers both the non-superconducting and superconducting regions. We found the unusual simultaneous occurrence of superparamagnetism and superconductivity with an x-independent Tc and a close correlation of the superconducting volume fraction with the magnetic cluster density and As-defect density. The finding demonstrates a close relationship among superconductivity, superparamagnetism, and defects, consistent with the previously proposed interface-mechanism, and offers a possible future path to higher Tc.
Two single crystalline samples with the same nominal composition of Rb0.8Fe2Se2 prepared via slightly different precursor routes under the same thermal processing conditions were investigated at ambient and high pressures. One sample was found superc
onducting with a Tc of ~31 K without the previously reported resistivity-hump and the other was unexpectedly found to be a narrow-gap semiconductor. While the high pressure data can be understood in terms of pressure-induced variation in doping, the detailed doping effect on superconductivity is yet to be determined.
We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one
starting at ~ 49 K, which has a low critical field ~ 4 Oe, and the other at ~ 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K phase detected will be discussed.
We report a $^{23}$Na and $^{75}$As nuclear magnetic resonance (NMR) investigation of Na$_{x}$FeAs series ($x=1$, 0.9, 0.8) exhibiting a spin-density wave (SDW) order below $T_{rm SDW}=45$, 50 and 43 K for $x=1$, 0.9, 0.8, respectively, and a bulk su
perconductivity below $T_capprox 12$ K for x=0.9. Below $T_{rm SDW}$, a spin-lattice relaxation reveals the presence of gapless particle-hole excitations in the whole $x$ range, meaning that a portion of the Fermi surface remains gapless. The superconducting fraction as deduced from the bulk susceptibility scales with this portion, while the SDW order parameter as deduced from the NMR linewidth scales inversely with it. The NMR lineshape can only be reproduced assuming an incommensurate (IC) SDW. These findings qualitatively correspond to the mean-field models of competing interband magnetism and intraband superconductivity, which lead to an IC SDW order coexisting with superconductivity in part of the phase diagram.
The magnetic properties of LiFeAs, as single crystalline and polycrystalline samples, were investigated. The lower critical field deduced from the vortex penetration of two single crystals appears to be almost isotropic with a temperature dependence
closer to that of two-gap superconductors. The parameters extracted from the reversible magnetizations of sintered polycrystalline samples are in good agreement with those from the single crystal data.
